Electrodeposition process with pretreatment in zinc phosphate solution containing fluoride



United States Patent 01 ifice 3,454,483 Patented July 8, 1969ELECTRODEPOSITION PROCESS WITH PRETREAT- MENT IN ZINC PHOSPHATE SOLUTIONCON- TAINING FLUORIDE Dennis B. Freeman, Harrow, England, assignor toHooker Chemical Corporation, Niagara Falls, N.Y., a corporation of NewYork No Drawing. Filed Apr. 29, 1965, Se r. No. 451,963

Claims priority, application Great Britain, Apr. 30, 1964,

18,043/ 64 Int. Cl. C23b 13/00; C23f 17/00; B01k 5/02 US. Cl. 204181 16Claims ABSTRACT OF THE DISCLOSURE An improved process forelectrophoretically painting metal surfaces wherein the metal to bepainted 1s first coated with an aqueous acidic zinc phosphate solutionwhich contains at least 0.1 grams per liter of fluoride ions. Thethus-coated metal surface is then rinsed with a dilute aqueous solutioncontaining hexavalent chromium and the thus-rinsed surface 'is thenelectrophoretically painted.

This invention relates to an improved process for painting metalsurfaces and more particularly, it relates to an improved method forpainting metal surfaces whereln the paint is applied by electrophoreticmethods.

In the treatment of various metal surfaces, 1t 1s frequently thepractice to provide on the metal surface a protective coating whichserves as a base for a subsequently applied paint. Frequently, suchpalnt-base coatings are phosphate coatings, such as zinc phosphatecoatings or the like. After the application of such phosphate paint-basecoatings, it is often the practice to apply a dilute aqueous solutioncontaining hexavalent chrom um ions to the phosphate coating. Suchchromate solutions enhance the paint coating which is subsequently applied to the metal surface. These phosphate paint base coatlngs andchromate rinse solutions and their methods of application are well-knownto those in the art. I

Recently, a great deal of Work has been done 1n developing processeswhereby water-thinned paint are applied to metal surfaces byelectrophoretic methods, WhlCh methods frequently involve the phenomenonof electroosmosis and electrolysis as well as electrophoresis. It hasbeen found that phosphate paint-base coatings, and particularly zincphosphate coatings, which have been applied prior to the use ofconventional painting techniques, are also beneficial when used prior tothe electrophoretic application of paint. In addition to the usualadvantages which are obtained by using the zinc phosphate materials aspaint-base coatings, it has been found that the Zinc phosphate coatinghas a higher resistance than the untreated metal surface. Thus, whenpaint is applied by electrophoresis, improved paint coverage on thesurface is obtained. The use of a chromate rinse over the phosphatecoating, prior to the electrophoretic application of the paint, has alsobeen found to be advantageous in the same manner as when such a rinse isused prior to the application of paint by conventional techniques.

Although the overall adhesion and quality of the paint coating isgenerally improved by the use of a chromate rinse over the zincphosphate coatings, in some instances it has been found that when suchrinse is used, minute depressions in the paint film applied byelectrophoretic application, are formed. In many instances, thesedepressions are actually holes which go completely through the paintfilm to the substrate and these defects have been found to beparticularly serious when the metal surface has been force dried afterthe application of the chromate rinse. Often, these depressions or holeshave been sufficiently bad that it has been necessary to completely omitthe chromate rinse after the application of the zinc phosphate coating.This is not, however, a satisfactory solution to the problem in that byeliminating the rinse, the advantages obtained by its use are also lost.

It is, therefore, an object of the present invention to provide animproved method for treating metal surfaces to provide a paint-basecoating thereon prior to the application of a paint by electrophoresis.

Another object of the present invention is to provide a pretreatment formetal surfaces, using a chromate rinse, prior to the application of apaint to the metal surface utilizing electrophoretic paintingtechniques.

A further object of the present invention is to provide an improvedprocess for painting metallic surfaces, whereby the paint is appliedusing electrophoretic methods.

These and other objects of the present invention will become apparent tothose skilled in the art from the description of the invention whichfollows.

Pursuant to the above objects, the present invention includes a processfor treating metal surfaces which comprises contacting the metal surfaceto be treated with an acidic zinc phosphate solution containing at least0.1 gram per liter of simple fluoride, calculated as F maintaining thesolution in contact with the metal surface for a period sufficient toform a zinc phosphate containing coating on the metal, contacting thethus-coated surface with a hexavalent chromium containing solutions and,thereafter, electrophoretically applying a paint to the thustreatedmetal surface. By thus utilizing a zinc phosphate solution whichcontains at least 0.1 gram per liter of simple fluoride to provide thepaint-base coating on the metal surface, the phosphate coating obtainedmay be rinsed with a hexavalent chromium containing solution and thenpainted using electrophoretic painting techniques, without the formationof depressions or holes in the paint film.

More specifically, the practice of the method of the present invention,a metal surface to be treated is contacted with an aqueous acidiccoating solution containing zinc phosphate and at least 0.1 gram perliter of a simple fluoride. Various metal surfaces may be treated by themethod of the present invention, such as ferrous metal surfaces, i.e.,iron or steel, including various alloys Wherein the predominantcomponent is iron, zinc or zinc alloy surfaces, wherein the predominantcomponent is zinc, including zinc coated ferrous surfaces, e.g., hotdipped galvanized iron or steel surfaces, and aluminum. The coatingsolutions are maintained in contact with the metal surface to be treatedfor a period'of time suflicient to effect the formation of the desiredzinc phosphate containing coating on the metal surface. Contact times ofabout /2 to about 3 minutes are typical although in some instanceseither lesser or greater contact times may be used, depending upon theamount of coating which it is desired to form on the surface.

' As has been indicated hereinabove, the phosphate coating solutionsused contain at least 0.1 gram per liter of a simple fluoride,calculated as F. The maximum :amount of the simple fluoride present hasnot been found to be critical, amounts up to the maximum solubility ofthe fluoride used in the phosphate solution being suitable, with amountsup to about 3-4 grams/ liter being typical. Various simple fluorides maybe used, provided the cation of the fluoride is not detrimental to themetal surface treated, the phosphate coating produced or thesubsequently applied paint coating. Exemplary of the fluorides which maybe used are the alkali metal fluorides, i.e., the fluorides of sodium,potassium, lithium, cesium, or rubidium, zinc fluorides, hydrofluoricacid, and the like. Of these, the preferred source of fluoride is sodiumfluoride or hydrofluoric acid and hereinafter, primary reference will bemade to the use of these materials. This is not, however, to be taken asa limitation as to the fluoride materials which may be used, but only asexemplary of these materials.

In many instances, it has been found to be advantageous also to includein the zinc phosphate coating solution a complex fluoride, in additionto the simple fluoride. Here again, various complex fluorides may beused, provided the cation of the complex fluoride is not detrimental tothe metal substrate, the phosphate coating produced or the paint coatingsubsequently applied. Exemplary of these complex fluorides which may beincluded in the composition are fluosilicates, fluoborates,fluotitanates, and the like. These may be added as the correspondingacid, the alkali metal salt, the zinc salt, or the like. Desirably, thecomplex fluoride added is a fluosilicate, preferably added asfluosilicic acid or sodium fluosilicate. Although varying quantities ofthe complex fluoride may be added to the phosphate coating solution, upto the maximum solubility of these fluorides in the solution, thepreferred ratio of complex fluoride, expressed as SiF to simplefluoride, expressed as F, is :1. This ratio of complex fluoride tosimple fluoride in the solution is a preferred ratio and is not to betaken as a limitation of the amount of complex fluoride in the solution,since amounts from 0% of the complex fluoride up to its maximumsolubility in the phosphate solution may be used.

The zinc phosphate coating solutions are aqueous acidic solutionscontaining phosphate ions and zinc ions, generally in amounts at leastsuflicient to form the dihydrogen phosphate with the phosphate ions, andare well-known to those in the art. Typically, such coating solutionsmay contain from about 0.5 to 2.5% by weight P0 ions although greaterand lesser amounts of the phosphate ions may be contained in thesolution in many instances. In addition to the simple fluoride and thecomplex fluoride, these solutions may also contain nickel, typically inamounts within the range of about 0.01 to about 0.4% by weight,calculated as Ni, copper ions, typically in amounts within the range ofabout 0.0003 to about 0.0005 by weight, and oxidizing ions such asnitrate and/or nitrite ions, the nitrate ions typically being present inamounts within the range of about 0.2 to about 1% by weight and thenitrite ions typically being present in amounts within the range ofabout 0.002 to about 0.01% by weight. Compositions of this type areexemplified by those described in U.S. Patent 2,835,617 and 2,591,479.These zinc phosphate coating solutions, are, however, merely exemplaryof those which may be used as other aqueous acidic zinc phosphatesolutions may also be used, provided they contain at least 0.1 gram perliter of the simple fluoride, as has been set forth hereinabove.

Contact of these coating solutions with the metal surfaces to be treatedmay be carried out in any convenient manner, as for example by spraying,immersion, flowing, or the like. In many instances, application of thesolutions by spraying is preferred so that primary reference hereinafterwill be made to the use of spraying techniques. Desirably, the solutionsare applied hot, temperatures within the range of about 40 to 80 degreescentigrade being typical, with temperatures within the range of about 45to about 70 degrees being preferred. After the desired contact of thesurfaces to be treated and the coating solution has been efiected, themetal surfaces are preferably rinsed with Water so as to remove any ofthe acidic coating solution which may remain on the surface. Preferably,a hot water rinse is used, with water temperatures within the range ofabout 50 to about 80 degrees centigrade being typical. As with theapplication of the phosphate coating solution, various contactingtechniques may be used, with rinsing by spraying being preferred.

After the hot water rinse, the phosphate coated surfaces are then rinsedin a hexavalent chromium containing solution. These hexavalent chromiumcontaining rinsing solutions are preferably aqueous solutions containinga source of hexavalent chromium, calculated as CrO typically in anamount within the range of about 0.03 to about 1% by weight of thesolution, and preferably in an amount within the range of about 0.07 toabout 0.5% by weight of the solution. Various water-soluble or Waterdispersable sources of hexavalent chromium may be used in formulatingthe rinsing solution, provided the anions and cations introduced withthe hexavalent chromium do not have a detrimental affect on either thesolution itself, the coated surfaces treated or the subsequently appliedpaint coating. Exemplary of hexavalent chromium materials which may beused are chromic acid, the alkali metal and ammonium chromates, thealkali metal and ammonium dichromates, the heavy metal chromates anddichromates, such as those of zinc, calcium, chromium, Fe+ magnesium andaluminum, and the like. The rinsing solution may be applied to thecoated metal surfaces using various techniques, including immersion,flooding, spraying and the like, with spraying techniques beingpreferred. Generally, it is preferred that the aqueous hexavalentchromium containing rinse solution is maintained at an elevatedtemperature while it is in contact with the phosphate coated metalsurface to be treated. Temperatures within the range of about 35 to 60degrees centigrade and contact times of up to about 60 seconds aretypical. If desired, these hexavalent chromium containing rinsingsolutions may also contain phosphate ions, preferably as orthophosphoric acid. Where phosphoric acid is included in the rinsecomposition, amounts up to about 1% by weight of the rinsing solutionare typical. It will, of course, be appreciated with regard to theconcentration of the hexavalent chromium and phosphate ions in the rinsesolution as well as the temperatures and contact times, that values bothgreater and lesser than those which have been indicated hereinabove, maybe used in some instances, the specific ranges which have been givenbeing merely exemplary of those which may be used.

Following the application of the hexavalent chromium containing rinsesolutions, the treated metal surfaces are, preferably again rinsed withwater so as to remove any of the acidic rinse solution which may remainon the surface. Thereafter, a water-thinned paint is applied to thetreated metal surfaces by electrophoresis. In the electrophoreticpainting process, the coated metal surface to be painted may be eitherthe anode or the cathode, depending upon the characteristics of thepaint which is used. The improvements obtained in using the method ofthe present invention have been found to be particularly striking whenthe coated metal surface is the anode, so that this method of operationis generally preferred.

The electrophoretic application of the paint may be carried out invarious ways, as are known to those in the art. Typically, the coatingsolutions utilized are dilute aqueous solutions, having a solids contentwithin the range of about 3 to 15 percent solids. As has been indicated,the metal to be coated is preferably the anode and the voltages used aretypically within the range of about 50 to 1000 volts (direct current).Typical current densities used and coating times required are,respectively, from about 0.1 to 7 amperes per square foot and from about10 seconds to about 2 minutes. Normally, the coating solution is atsubstantially room temperature, i.e., about degrees centigrade, butelevated temperatures, e.g., 30 to 40 degrees centigrade or even higher,may be used if desired. The paint applied using these techniques areWater-thinned resin paints which are, typically aqueous solutions basedon synthetic resins such as alkyd resins, acrylic polymers, melamineresins, and the like. These aqueous resin solutions generally have a pHof about 9 and the solvent used is either water or an aqueous alcoholicmixture.

In carrying out the method of the present invention, the metal surfaceto be treated, such as a ferrous metal surface, is first cleaned.Although any conventional metal cleaning composition and method may beused, in many instances it has been found desirable to utilize acleaning solution containing an alkali metal silicate and alkali metalmetaborate, such as that disclosed in British patent specification No.932,970. Exemplary of such a composition is one having the followingcomposition:

Components: Parts by weight Sodium metasilicate 616 Sodium metaborate300 Titanium activator containing 1.5% titanium and prepared inaccordance with US. Patent Such cleaning compositions may be applied invarious ways, such as by immersion, flowing, spraying, or the like, withspraying techniques being preferred.

After the cleaning of the metal surface has been completed, it may, ifdesired, be rinsed with water to remove any of the alkaline cleaningsolution which may remain on the surface. The cleaned metal surface isthen contacted, preferably by spraying, with an aqueous acidic zincphosphate solution containing at least 0.1 gram per liter of a simplefluoride, until the desired zinc phosphate coating is formed on themetal surface. The coated metal may then be rinsed in water, followed bya rinse with dilute chromic acid or a dilute solution of chromic acidand phosphoric acid. If no intermediate operations or inspection arerequired, the rinsed metal surface may be passed directly through thepainting tank, wherein a paint coating is applied -by electrophoresis,without any intermediate drying. In this case, however, it may bedesirable to water rinse the metal surface after the chromate rinse soas to avoid contamination of the paint. In the electrophoreticapplication of the paint, the paint tank itself may serve as the cathodewhile the parts to be coated, as the anode, may be introduced into thetank by means of an energized conveyor. After the application of thepaint, the painted surfaces may then be subjected to whatever heating isnecessary to effect the necessary drying and/or curing of the paintlfim.

In order that those skilled in the art may better understand the presentinvention and the manner in which it may be practiced, the followingspecific examples are given. These examples are, however, merelyexemplary of the process of the present invention and are not to betaken as a limitation thereof. In these examples, unless otherwiseindicated, temperatures are given in degrees centigrade.

Example 1 Ferrous metal panels were cleaned using the compositions andprocedures as set forth in British patent specification No. 932,970.Following the cleaning, the panels were rinsed twice in cold water. Thepanels were then phosphate-coated by spraying with a solution containing1.8 grams per liter N 2.3 grams per liter zinc, 5.9 grams per liter PO0.3 grams per liter nickel, 0.18 grams per liter sodium and 0.07 gramsper liter N0 1.2 grams per liter Na SiF and 0.3 grams per liter NaF.This solution was at a temperature of about 50 degrees centigrade andthe solution was sprayed on the panels for about 1 minute. Following theapplication of the phosphate coating, the panels were water rinsed andthen sprayed for 30 seconds with an aqueous solution containing 0.09grams per liter chromic acid and 0.06 grams per liter phosphoric acid,which solution was at a temperature of about 40 degrees centigrade.These panels were then dried in an oven at a temperature of about 120degrees centigrade for 5 minutes. The thusdried panels were then paintedby electrophoresis with a water-based primer paint. The paint was anaqueous composition of a high acid number, above 50, phenol modifiedalkyd resin containing added hydroxyl substituents, which compositioncontained about 10% solids. The paint had a pH of about 7.3 and was at atemperature of about .20 degrees Centigrade. With the panels as theanode, a direct current was applied for about two minutes at a voltageof about 160 volts to deposit the paint film. During this time, thecurrent density, which was initially about 5 amps/square foot, droppedto about 0.5 amp/square foot. The resulting paint coating was smooth,showed no evidence of depressions or pin-holes and gave excellentresults when tested in the 5% salt spray, humidity, knife blade adhesionand standard deformation tests.

The procedure of the proceeding examples is repeated using a zincphosphate solution containing hydrofluoric acid and fluosilicic acid inplace of the respective sodium salt, and using a chromate rinsecontaining about 0.13 grams per liter chromic acid, to obtainsubstantially the same results.

Example 2 By way of comparison, the procedure of Example 1 was repeatedwith the exception that the zinc phosphate coating solution usedcontained no sodium fluoride or sodium fluosilicate. The paint coatingobtained on the panels which were coated by this process was found tohave numerous pin-holes, many of which extended through the paint filmto the substrate.

Example 3 The procedure of Example 1 was repeated with the exceptionthat the zinc phosphate coating solution used contained about 0.3 gramper liter of hydrofluoric acid instead of the sodium fluorsilicate andsodium fluoride. The painted panels obtained by using this procedurewere found to be substantially the same as those obtained in Example 1,showing no evidence of pin-holing or other defects in the film.

While there have been described various embodiments of the invention thecompositions and methods described are not intended to be understood aslimiting the scope of the invention, as it is realized that changestherewithin are possible and it is intended that each element recited inany of the following claims is intended to be understood as referring toall equivalent elements for accomplishing the same results insubstantially the same or equivalent manner, it being intended to coverthe invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. In a process for painting a metal surface wherein the surface iscoated with a phosphate coating, the coating rinsed with a dilutesolution containing hexavalent chromium and, thereafter, paint isapplied to the rinsed surface by means of electrophoresis, theimprovement which comprises forming the phosphate coating on the surfaceby contacting the surface with an aqueous acidic zinc phosphate solutionwhich contains at least 0.1 grams per liter of a simple fluoride,calculated as F".

2. The process as claimed in claim 1 wherein the zinc phosphate coatingsolution also contains a complex fluoride.

3. The process as claimed in claim 2 wherein the ratio of complexfluoride, calculated as SiF to simple fluoride calculated as F- is about5:1.

4. The process as claimed in claim 3 wherein the complex fluoride andthe simple fluoride are added to the zinc phosphate solution as sodiumfluosilicate and sodium fluoride respectively.

5. A process for treating metal surfaces which comprises contacting themetal surface to be treated with an aqueous acidic zinc phosphatesolution containing at least 0.1 grams per liter of a simple fluoride,calculated as F, maintaining the phosphate solution in contact with themetal surface for a period sufiicient to form a zinc phosphatecontaining coating on the metal surface, rinsing the thus-coated surfacewith a hexavalent chromium containing solution and, thereafter,electrophoretically applying paint to the thus-treated metal surface.

6. The method as claimed in claim wherein the zinc phosphate solutionalso contains a complex fluoride.

7. The method as claimed in claim 6 wherein the ratio of complexfluoride, calculated as SiF to simple fluoride, calculated as F, isabout 5:1.

8. The process as claimed in claim 7 wherein the complex fluoride andthe simple fluoride are added to the zinc phosphate solution as sodiumfiuosilicate and sodium fluoride, respectively.

9. The process as claimed in claim 8 wherein the zinc phosphate coatingsolution also contains at least 0.01 gram per liter of nickel,calculated as Ni.

10. The process as claimed in claim 5 wherein the coated metal surfaceis the anode during the electrophoretic application of the paint.

11. The process as claimed in claim 7 wherein the coated metal surfaceis the anode during the electrophoretic application of the paint.

12. The process as claimed in claim 9 wherein the coated metal surfaceis the anode during the electrophoretic application of the paint.

13. The process as claimed in claim 12 wherein prior to contacting themetal surface to be treated with the zinc phosphate coating solution,the surface is cleaned by contacting it with an aqueous cleaningsolution'containing an alkali metal silicate and an alkali metalmetaborate.

14. An article having a painted metal surface formed in accordance withthe process of claim 5, which surface is characterized by having thereona paint film which is substantially free of holes.

15. An article having a painted metal surface formed in accordance withthe process of claim 9 which surface is characterized by having thereona paint film which is substantially free of holes.

16. An article having a painted metal surface formed in accordance withthe process of claim 13, which surface is characterized by havingthereon a paint film which is substantially free of holes.

References Cited UNITED STATES PATENTS 2,312,855 3/1943 Thompson 148-6152,477,841 8/1949 Ward 1486.l5 2,665,231 l/l954 Amundsen et al. 1486.152,935,423 5/1960 Kapfer et al. 1486.15 3,060,066 10/1962 Ross et al.1486.15 3,261,723 7/1966 Craig 148-615 3,364,081 1/1968 Forsbcrg 1486.152,438,013 3/ 1948 Tanner 204 2,631,951 3/1953 Chester 204-35 2,648,6258/1953 Buser 204--35 2,825,682 3/1958 Missel et al. 204-382 3,106,48410/1963 Miller 20435 3,230,162 1/1966 Gilchrist 204-181 FOREIGN PATENTS768,443 2/ 1957 Great Britain.

JOHN H. MACK, Primary Examiner.

E. ZAGARELLA, JR., Assistant Examiner.

. U.S. Cl. X.R. 2043

